1. Field of the Invention
This invention relates to an image forming apparatus, such as a laser printer, for forming an image on an image bearing member and, more particularly, to an image forming apparatus having a detachably mounted process unit.
2. Description of the Related Art
Generally, in a conventional image forming apparatus, a latent image is first formed on an image bearing member. A developing device then develops the latent image, using a toner to make the latent image visible.
In recent image forming apparatus designs, several elements of the apparatus have been contained in one casing, so as to form a replaceable unit (herein referred to as a "processing unit"). For example, the image bearing member and the developing device having a toner supply device have been contained in a single processing unit, which may be detached from the apparatus and replaced with a new processing unit upon expiration of its service life. U.S. Pat. No. 3,985,436 discloses an image forming apparatus wherein the image bearing member, the developing device and the cleaning device are contained in a single unit. This unit is replaced with a new unit when the service life of the image bearing member expires. Thus, all the devices contained within the single unit are exchanged simultaneously.
In the above-described image forming apparatus, the user must somehow be informed as to when the service life of the processing unit expires, in order to maintain high quality imaging. In other words, the image forming apparatus must include a service life management device.
In a conventional image forming apparatus, the service life management device is a counter for detecting the amount of use of the processing unit. The processing unit is then replaced when the counter indicates a certain predetermined number. Also, in order to perform replacement of the processing unit, a message indicating that the processing unit should be changed may then be displayed on a display unit of the laser printer when the service life of the processing unit expires. However, since the display area of the display unit is generally small, the message indicating that the processing unit should be changed must be displayed with a symbol or short sentence. The operator may not accurately understand the content of the message. Thus, the user may not replace the processing unit immediately after the message is displayed on the display unit of the laser printer. When the replacement of the processing unit is not performed at an optimal timing, accurate image forming may not be performed.
Also, an non-expert user performs the replacement of the processing unit in accordance with instructions (an explication of the operation and a picture) described an operation manual. It is, however, cumbersome for the non-expert user to find desired instructions from the operation manual when the non-expert user replaces with the processing unit.